1. Technical Field of the Invention
This invention relates to telecommunication systems and, more particularly, to a method of delivering Short Message Service (SMS) messages to a mobile station via a satellite air interface radio link in a radio telecommunications network.
2. Description of Related Art
FIG. 1 is a simplified block diagram of an existing satellite system based on a Global System for Mobile Communications (GSM) radio telecommunications network 10 illustrating the network nodes involved in delivering SMS messages to a mobile station 13. A satellite link is shown to include a satellite gateway 11, a communication satellite 12, and the MS 13 which may be partially shielded within a structure 14. Within the gateway 11, a Mobile Switching Center/Visitor Location Register (MSC/VLR) 15 is connected to a Base Station Subsystem (BSS) 16 via an A-interface 17. The BSS is modified for use with satellites, and is connected to a Radio Frequency Terminal (RFT) 18 via, for instance, a modified Abis interface 19. The RFT 18 includes an antenna system for tracking and communicating with the satellite 12. The satellite 12 provides an air interface link from the gateway 11 to the MS 13. A feeder link 21 (from the gateway 11 to the satellite 12) typically operates in the Ku-band. A downlink 22 (from the satellite to the MS) typically operates in the Ku-band or L-band. Different bands have different propagation characteristics depending, for instance, on rain attenuation. There is some attenuation of the signal on all bands if the mobile station antenna is not deployed, or if the mobile station 13 is located inside a building.
The network 10 also includes a SMS Service Center (SMS-SC) 23, a Short Message Gateway Mobile Switching Center (SM-GMSC) 24, a Home Location Register (HLR) 25, and a Short Message Interworking Mobile Switching Center (SM-IWMSC) 26 to handle outgoing messages from the network 10.
The air interface link includes a plurality of logical channels such as traffic channels, broadcast channels, dedicated control channels, and various associated channels. In the normal GSM architecture, there is also a single paging channel which accommodates various paging methods using different algorithms such as, for example, global paging, repeated paging, and so forth. Paging over a satellite link, however, poses additional problems which are not typically encountered by paging systems in cellular telecommunications networks. When paging over a satellite link, there is increased attenuation of the signal due to the distance the signal must travel, water vapor in the air, and various power limits imposed on the satellite transceiver. By the time the paging signal reaches the ground, it is likely that the signal will not penetrate walls or other obstructions such as wet foliage.
To address this problem in the past, satellite networks have typically either utilized a higher power level on the satellite, or a separate logical channel has been defined which utilizes different channel coding. Utilizing higher power on the satellite is a problem because satellites are typically power-constrained. The available power on the satellite is one of the major constraints on its throughput, and high power cannot be used for every page because this would decrease the amount of total traffic that the satellite can handle. Therefore, higher power levels are generally utilized sparingly. If a separate logically channel is defined and utilized for paging, it creates other problems because this method is less effective than raising the power level, and less information can be carried by the paging signal since additional coding must be used on the channel.
Satellite paging systems also suffer from reliability problems caused, once again, by the attenuation of the signal and the inability to page the mobile station when the mobile station is in a disadvantaged mode (i.e., the mobile station is in a shielded location and/or the mobile station antenna is not raised). This can lead to lost messages or, if the network takes corrective action as described below, the same message may be delivered more than once.
In order to overcome the disadvantage of existing solutions, it would be advantageous to have a method of paging mobile stations in a satellite radio telecommunications network which uses high power paging only when needed, and conserves satellite power by using lower power at other times. In addition, it would be advantageous for such a paging system to reliably deliver SMS messages to the paged mobile station without delivering the same message more than once. The present invention provides such a method.